This patch implements drag-and-drop for w32 for files and text, using C,
not C++.
This should work from Windows 95, but I can't test it.  Because of this,
the prior implementation with WM_DROPFILES has been removed with '#if
0', and can probably be completely removed.

I tested with mingw-w64 and mingw on Windows 11.

Text is inserted at point position, not at drop position.  I don't know
how this works on other platforms.